1. Technical Field
The present invention relates to a movement controller for moving a mobile body of a machine tool to a designated position, a machine tool provided with such a movement controller, and a method for moving the mobile body.
2. Description of the Related Art
In a machine tool such as an automatic lathe, when machining one workpiece sequentially, and using a plurality of tools, it is common practice, as shown in FIG. 18, to move a mobile body 40, i.e., a tool post provided with a plurality of cutting tools, in a U-shaped or L-shaped path relative to a spindle 50 by using a servo motor. To accomplish this movement, the mobile body 40 is moved and stopped repeatedly in such a manner that the mobile body 40 is first moved by an X-axis driving unit, then by a Y-axis driving unit, and again by the X-axis driving unit.
FIG. 19a is a diagram showing in simplified form the moving path of the mobile body (for example, the tool post) when the mobile body moves in a U-shaped path as shown in FIG. 18. As shown, the mobile body first moves rectilinearly from a first position “a” (Xa, Ya) to a first direction change point “b” (Xb, Ya), then moves rectilinearly from the point “b” to a second direction change point “c” (Xb, Yc), and finally moves from the point “c” to a second position “d” (Xd, Yc). Here, when the mobile body moves from the point “a” to the point “b” and from the point “c” to the point “d”, the mobile body is driven by the X-axis driving unit, and when the mobile body moves from the point “b” to the point “c”, the mobile body is driven by the Y-axis driving unit. The points “b” and “c” are the points through which the mobile body has to pass when moving from the point “a” to “d” in order to avoid an obstacle located therebetween. That is, the mobile body does not move rectilinearly from the point “a” to the point “d”, but always passes through direction change points that are not located on the straight line jointing the points “a” and “d”.
FIG. 19b is a diagram showing how the driving speeds of the X-axis and Y-axis driving units as the first and second driving units capable of driving in the mutually orthogonal first and second axis directions, i.e., the X-axis and Y-axis directions, change with respect to time (i.e., the change over time of the moving speed along each axis direction) when the mobile body moves as shown in FIG. 19a. When moving the mobile body, each driving unit is controlled to operate at its maximum performance in order to shorten the overall moving time as much as possible. More specifically, first, one of the driving units (in the illustrated example, the X-axis driving unit) drives the mobile body from the point “a” by accelerating it with a predetermined or rated first maximum acceleration, i.e., with a minimum time constant Tx1 and, once a first maximum moving speed Fx is reached, the mobile body is driven constantly at Fx for a predetermined length of time; then, as the mobile body approaches the point “b”, the mobile body is decelerated with a predetermined or rated first maximum deceleration, i.e., with a minimum time constant Tx2, until it comes to a stop at the point “b”. Next, in a manner similar to the X-axis driving unit, the other driving unit (in the illustrated example, the Y-axis driving unit) drives the mobile body from the point “b” by accelerating it with a predetermined or rated second maximum acceleration, i.e., with a minimum time constant Ty1 and, once a second maximum moving speed Fy is reached, the mobile body is driven constantly at Fy for a predetermined length of time; then, as the mobile body approaches the point “c”, the mobile body is decelerated with a predetermined or rated second maximum deceleration, i.e., with a minimum time constant Ty2, until it comes to a stop at the point “c”. Finally, the X-axis driving unit moves the mobile body from the point “c” to the point “d” in the same manner that it was moved from the point “a” to the point “b”.
Many attempts have been made to shorten the overall moving time including the move-and-stop operations described above. To shorten the overall moving time, it is advantageous to drive both driving units simultaneously during a certain part of the time, rather than driving one or the other of the driving units selectively. Japanese Unexamined Patent Publication No. H09-262742, for example, discloses a feed control method and apparatus in which feed motions are performed simultaneously in two axis directions. In this control method, when the tool is being fed only in the X-axis direction, the tool is also moved in the Z-axis direction before the tool reaches a designated X coordinate, thereby attempting to shorten the machining time. On the other hand, a rapid-feed control method for a mobile body of a machine tool is described in Japanese Unexamined Patent Publication No. H07-134608. This control method attempts to shorten the moving time by reducing the acceleration and deceleration time constants.
When moving a mobile body, provisions often have to be made to avoid interference with other members during the movement, while at the same time, achieving a reduction in the moving time. For example, Japanese Unexamined Patent Publication No. H11-104934 discloses a control method and apparatus for shortening the time it takes to move a tool changer to a tool change position. In this control method, an approach position P2 is set through which the tool changer must pass in order to avoid interference with other members when the tool changer moves from the current position P1 to the tool change position P3, and control is performed so that the tool changer moves without once stopping at P2.
When moving a mobile body such as a tool post in a U-shaped path, the mobile body is caused to stop and then restart at each corner of the letter U, i.e., at each direction change point to change direction from the first axis direction to the second axis direction or vice versa. More specifically, the mobile body is decelerated just before it reaches the corner corresponding to the end of the axis direction along which it is being moved; then, after once stopping at the corner, the mobile body is accelerated to move along the other axis direction. Generally, in order to minimize the moving time, the deceleration and acceleration operations are performed with the maximum deceleration and acceleration that the mobile body driving unit can provide, i.e., with the minimum time constant. At this time, a large mechanical impact such as vibration is applied or damage is caused to the ball screw and other components of the driving unit, and this has been one of the causes that shorten the service life of the components. The method described in Japanese Unexamined Patent Publication No. H09-262742 aims to shorten the moving time, but with this method, the mobile body cannot be moved so as to pass through the second point (the point “c” in FIG. 19a) provided in order to avoid interference; besides, no description is given about the way to reduce the mechanical impact. With the method described in Japanese Unexamined Patent Publication No. H07-134608, the mechanical impact may become more pronounced, since the time constant for the acceleration and deceleration of the mobile body is reduced. On the other hand, with the method described in Japanese Unexamined Patent Publication No. H11-104934, the moving time of the mobile body, i.e., the tool changer, can be reduced while preventing the mobile body from interfering with other members, but since the mobile body is caused to abruptly change direction at the point (P2) through which the mobile body passes in order to avoid interference, it is considered that, at P2 also, the X-axis and Y-axis driving units each apply a rapid acceleration or deceleration to the mobile body. In Japanese Unexamined Patent Publication No. H11-104934 also, there is no mention of a way to reduce the mechanical impact.